deb/apk: make local repodata deterministic (#119)

Part of #117. Local generated repodata must be byte-identical across the two no-affinity replicas and across every regeneration, so apt/apk never hit a checksum mismatch between an index's advertised hash and the bytes actually served. This does the deb+apk half (the rpm half landed in #118).

How:
- Derive the deb `Release` `Date:` from the newest persisted `created_at` (RFC1123Z, UTC) instead of `time.Now()`; carry `created_at` through the deb metadata SELECT and `DebMetadata`. An empty repo falls back to the Unix epoch. This also stops `Date:` running ahead of wall clock.
- Pin the apk `APKINDEX` tar header `ModTime` to the Unix epoch instead of the zero-value `time.Time`, so it is never wall-clock derived.
- Give both list queries a genuine total order by adding a `file_path` tiebreak (name/version/arch is not unique).
- Add guard tests: deb generators byte-identical across generations; the `Release` checksum/size matches the served `Packages`/`Packages.gz` bytes (the exact apt invariant); `Date:` pinned to `created_at`; apk index byte-identical and tar `ModTime` pinned to epoch.

Reviewed-on: #119
Co-authored-by: unkin-agent <unkin-agent@unkin.net>
Co-committed-by: unkin-agent <unkin-agent@unkin.net>

internal/database/alpine_metadata.go

@@ -41,7 +41,7 @@ func (db *DB) ListAlpineMetadataEntries(ctx context.Context, repoName string) ([
 			depends, provides, install_if
 		FROM alpine_metadata
 		WHERE repo_name = $1
-		ORDER BY name, version, arch
+		ORDER BY name, version, arch, file_path
 	`, repoName)
 	if err != nil {
 		return nil, err